A technology relating to this type of fuel injection device is disclosed in Japanese Patent No. 2885076 and International Publication No. 00/55496. A fuel injection device in the related technology will be described with reference to FIG. 12.
At a time when fuel is injected, an injection control chamber 3 is connected via an orifice 35 to a drain 22 by means of an injection control valve 7, for reducing the pressure inside the injection control chamber 3 to a level close to atmospheric pressure. Then, because a force acting on a needle 51 toward the injection control chamber 3 side exceeds a force toward an injection hole 23 side, the needle 51 is moved toward the injection control chamber 3 side, thereby opening the injection hole 23. As a result, the fuel stored in a fuel storage 52 is injected from the injection hole 23 into a combustion chamber of an internal combustion engine (not illustrated).
In addition, a booster control chamber 102 is connected to the drain 22 by means of a booster control valve 8, thereby reducing the pressure inside the booster control chamber 102 to a level close to atmospheric pressure. When the pressure inside the booster control chamber 102 reaches the level, a booster piston 10 is actuated to thereby increase the pressure of fuel in a booster chamber 103, which in turn increases the pressure of fuel stored in the fuel storage 52. In this manner, the fuel stored in the fuel storage 52 can be pressurized and injected at an increased pressure. It should be noted that because the booster chamber 103 communicates with the injection control chamber 3 via an orifice 60, the pressure inside the booster chamber 103 increased by the booster piston 10 is supplied to the booster control chamber 3 via the orifice 60 in addition to being supplied to the fuel storage 52. Because of this, even when the booster control chamber 102 is connected to the drain 22 in a state where the injection control chamber 3 is not connected to the drain 22, there is prevented movement of the needle 51 toward the injection control chamber 3 side, which would result in the opening of the injection hole 23.
Meanwhile, when the injection of fuel is terminated, communication between the injection control chamber 3 and the drain 22 is interrupted by means of the injection control valve 7. Then, because fuel pressure is supplied from a common accumulator 2 via a check valve 59 and the orifice 60 to the injection control chamber 3, the force exerted on the needle 51 toward the injection hole 23 side becomes greater than the force toward the injection control chamber 3 side, which moves the needle 51 toward the injection hole 23 side to thereby close the injection hole 23. Subsequently, fuel is supplied from the common accumulator 2 via the check valve 59 to the fuel storage 52 and the booster chamber 103.
In addition, when the pressure inside the booster control chamber 102 is increased to a common rail pressure by connecting the booster control chamber 102 to the common accumulator (common rail) 2 by means of the booster control valve 8, the pressures above and below the booster piston 10 are balanced as appropriate, so that the booster piston 10 actuated by the force of a spring 98 is returned to its initial position.
Other related techniques are disclosed in Japanese Patent Publication No. Sho 47-38648, International Publication No. 01/14727, U.S. Pat. No. 6,427,664, and SAE TECHNICAL PAPER SERIES 960107, 1996/2 entitled “Injection Rate Shaping Technology with Common Rail Fuel System (ECD-U2)” by Kenji Funai et al.
In the fuel injection device shown in FIG. 12, when the injection of fuel is terminated, the pressure of fuel supplied from the common accumulator 2 via the check valve 59 and the orifice 60 to the injection control chamber 3 pushes the needle 51 toward the injection hole 23 side. However, the pressure of fuel supplied from the common accumulator 2 via the check valve 59 to the fuel storage 52 also pushes the needle 51 toward the injection control chamber 3 side, which hampers movement of the needle 51 for closing the injection hole 23. Accordingly, there is a problem in that when the needle 51 closes the injection hole 23, performance of terminating the injection of fuel is degraded, and a state of atomization of injected fuel is in turn deteriorated.
Further, in the fuel injection device shown in FIG. 12, the pressure inside the booster chamber 103 increased by the booster piston 10 is supplied via the orifice 60 to the injection control chamber 3, in addition to being supplied to the fuel storage 52. Because the injection control chamber 3 communicates via the orifice 35 with the drain 22 when fuel is injected, a portion of the fuel increased in pressure by the booster piston 10 is discharged through the injection control chamber 3 to the drain 22, which results in a problem that difficulty is encountered in effectively pressurizing and injecting the fuel stored in the fuel storage 52 by means of the booster piston 10.
Still further, during low-load operation of an internal combustion engine, desirably, a fuel injection rate is suppressed in an initial phase of injection, in view of reducing combustion noise. On the other hand, during high-load operation of the internal combustion engine, in view of securing high power, it is desired that a high injection rate be rapidly attained rather than suppressing the fuel injection rate in the initial phase of injection. As such, it is desired that characteristics of fuel injection rate be able to be changed appropriately in accordance with an operation state of an internal combustion engine.
It is an advantage of the present invention to provide a fuel injection device which exhibits improved performance in terminating fuel injection when a needle closes an injection hole. It is another advantage of the present invention to provide a fuel injection device capable of efficiently performing operation of injecting fuel pressurized by a booster piston. Further, it is still another advantage of the present invention to provide a fuel injection device capable of appropriately changing characteristics of fuel injection rate in accordance with an operation state of an internal combustion engine.